Instrumentation displays in aircraft have been predominately incorporated into panel displays. A pilot flying an aircraft with such displays has to look down from a scene external to the aircraft to the panel to view instrumentation displays and, therefore, can not simultaneously view the external scene as well as the panel instrumentation displays. Furthermore, even if simultaneous viewing were possible, the external scene and the panel displays are at different focal lengths from the pilot. A pilot would have to refocus to read the displays after observing the external scene or to observe the external scene after reading the displays and could not, therefore, simultaneously interpret the displays and the external scene.
As a result, Head Up Displays (HUDs) were developed so that a pilot could simultaneously view an image of selected display information and an external scene through a window, without having to refocus. The HUDs utilize a lens to collimate light rays from an image of the selected display information so that the light rays appear to be projected from a location distant from the pilot. That image is superimposed on the view of an external scene the pilot has through a fixed window, thereby allowing simultaneous viewing of the selected display information overlaying the external scene without requiring the pilot to refocus. HUDs, however, only provide the pilot with a fixed, limited field of view as defined by the fixed window.
In a further development, Helmet Display systems allow simultaneous viewing of selected display information and an external scene corresponding to the pilot's field of view. In some systems, the visor on the pilot's helmet acts as a holographic collimating combiner. The pilot views an external scene through his visor and the image of the selected display information is superimposed on the visor. Systems which allow simultaneous viewing of selected display information and the external flight environment during good visibility conditions, therefore, are known.
During low visibility conditions, such as the night-time, however, such systems are not yet available. Currently, during such conditions, Night Vision Goggles are placed in front of the eyes of the pilot to intensify the light from a low visbility scene external to the aircraft, allowing a pilot to view the scene directly and superimposed over the dim visual scene. The goggles, however, can not be used with existing helmet display systems because they do not fit into the limited space between the pilot's eye and the helmet display visor. A system which allows a pilot to simultaneously view combined images of display information and external flight environment during low visibility conditions, therefore, is needed.